In recent years, development of computers and development of networks are remarkable, whereby various kinds of information data such as character data, image data, voice data and the like are handled in the computers and the networks. Therefore, contents business for dealing in so-called digital contents such as digital still images, digital moving images, digital music and the like on the network becomes remarkably active.
Incidentally, since the target of the contents business is the digital data, such a fact brings about an environment that a duplicate of homogeneous data can be easily generated. For this reason, in order to protect the copyright of the data which can be easily duplicated in the above environment, a process for embedding copyright information and/or user information in the image data and/or the voice data as a digital watermark is often executed.
Here, it should be noted that a digital watermark technique is the technique for executing a predetermined process to the image data and/or the voice data so as to concealedly or secretly embed another information in the image data and/or the voice data. In this connection, if the digital watermark is extracted from the digital data, then the copyright information, the user information, discrimination information and the like can be acquired from the extracted digital watermark, whereby it is possible to follow up illegal copy and illegal use.
By the way, in the commercially available digital watermark technique, any algorithm thereof is not published ordinarily. This is because security in the digital watermark technique is based on the premise that the algorithm, information concerning information embedded location, and the like are concealed. Here, such a digital watermark technique is called a private key digital watermark here.
Incidentally, the digital watermark method capable of publishing the algorithm of a digital watermark (digital watermark algorithm), the embedment location of digital watermark information and the like is disclosed in, for example, U.S. Pat. No. 6,807,285. More specifically, in this method, the digital contents are wholly subjected to an error correction encoding process, whereby, even if the embedded information at the published location is damaged, the damaged embedded information can be restored from the whole of the digital contents.
Moreover, in the above digital watermark method, error correction is executed without depending on the digital watermark algorithm, whereby it is possible to publish the digital watermark algorithm. That is, the digital watermark method by which the digital watermark algorithm and the information-embedded location can be published is called a published key digital watermark here.
More specifically, in the published key digital watermark disclosed in U.S. Pat. No. 6,807,285, the watermark information is embedded in a first area of an image by the published digital watermark method, and an error correction code (that is, the information used for restoring the watermark information) is embedded in a second area different from the first area. Here, although it is possible to publish the first area in which the watermark information is embedded, it is necessary to conceal the second area in which the error correction code is embedded.
Moreover, the published key digital watermark disclosed in U.S. Pat. No. 6,807,285 is used to embed various information such as copyright information, user information and the like. Here, with respect to the same digital content, the user information is inevitably different if the user is different. In contrast, the copyright information is always the same with respect t the same digital content. However, any method which is specialized only to embed information such as the copyright information which is common to the certain content is not proposed conventionally.